This invention relates to a power steering control apparatus used for an automobile etc., and in particular to a power steering control apparatus for improving safety during high speed running upon the occurrence of an abnormal condition at least in a current feedback loop and capable of preventing a secondary failure.
Generally, it is required that a power steering control apparatus for an automobile reduce (lighten) the manual steering power required in proportion to the steering load while increasing the manual steering power required in proportion to the car speed so as to make the steering operation safe during high speed running.
For this, it has been generally followed in such power steering control apparatus that the coils of a proportional solenoid have flowing therethrough current in proportion to a car speed via electrical wiring within the car from an electrical control system in the car, thereby providing a proper hydraulic reaction force for a steering the car by actuating a hydraulic valve in approximate proportion to said flowing current.
However, disadvantages arise when a failure of the proportional solenoid or the electrical wiring gives rise to a shortcircuit between the coils, a ground fault a shortcircuit between the wiring lines, or when a closed loop for current control within the electrical control system fails, resulting in the dissapearance of a current detection signal so that no current control can be performed. Therefore, excessive current flows through the coils of the proportional solenoid so that the coils may be over-heated and burned, or a power transistor in a solenoid driving circuit may be broken down. Also, since the driver can steer the handle too lightly during high speed running, the manual steering operation becomes unstable so that the driver feels uneasy, resulting in a disadvantageous and dangerous condition.
Japanese Patent Application Laid-open No. 58-188751 discloses a protection circuit in which a chopper circuit for driving a power steering motor is disconnected in the event that an input/output signal line for an angular sensor serving for the reciprocal rotation of a power steering motor is broken.
Japanese Patent Application Laid-open No. 56-146473 discloses a power steering control apparatus in which an output power transistor is prevented from being destroyed, overheated, or burned even when a linear solenoid as a load is shorted and the wiring is grounded, while a steering condition is maintained at a low speed.
Japanese Patent Application Laid-open No. 57-178972 discloses a speed sensitive power steering apparatus in which when a change gear selects a gear position other than that of the first gear and the reverse gear, the absence of a car speed signal is detected for a failsafe operation.